Since traditional fossil energy resources are limited, research is being directed towards the use of alternative renewable fuels. One of the approaches is the conversion of vegetable oils and animal fats into biodiesel. Vegetable oils and animal fats are comprised of complex mixtures of triglycerides (TGs) and other relatively minor components, such as free fatty acids (FFAs), gums, waxes, etc. Biodiesel is usually made through a chemical process called transesterification, whereby TGs react with methanol in the presence of a catalyst to produce a complex mixture of fatty acid alkyl esters (biodiesel) and glycerol.
Many of the commercial biodiesel that are currently produced in the U.S. come from the transesterification of soybean oil using homogeneous base (such as NaOH or KOH) catalyzed processes. Alkali cations are removed after the transesterification reaction as alkali soaps in glycerol phase. An acidic neutralization step with aqueous acid is required to neutralize these salts. Even though homogeneous catalyzed biodiesel production processes are relatively fast and show high conversions, usage of homogeneous base catalyst suffers from the formation of undesirable side reaction such as saponification which creates problems in product separation and ultimately lowers the ester (biodiesel) yield.
In order to minimize problems associated with the homogeneous catalytic processes, attempts have been made to develop heterogeneous catalyst systems in transesterification of triglycerides. Solid base catalysts are used to replace alkaline homogeneous catalysts, to minimize soap formation, separation, corrosion and environmental problems. At the laboratory scale, many different heterogeneous catalysts have been reported, including MgO, hydrotalcites, zeolites loaded with sodium oxide, Li/CaO, KF/ZnO, mixed metal oxides (Al2O3—SnO, Al2O3—ZnO), Zn/I2, mixed oxide of zinc and aluminum, and potassium loaded alumina. Catalytic activities of the heterogeneous base catalysts in the transesterification of soybean oil show a correlation with their corresponding basic strengths. Although alkali metal-containing catalysts show strong basicities, alkali metal ions are easily dissolved in the reaction media. Thus, reaction proceeds according to homogeneous mechanism. Other solid metal oxides such as those of tin, magnesium, and zinc are known heterogeneous catalysts but again function according to a homogeneous mechanism leading to metal soaps or metal glycerates.
Much work has focused on the preparation of solid catalysts possessing strong basic sites. Strong basic sites are generated by removal of water or acidic gas molecules by pretreatment at high temperatures. These basic sites are fragile and can be easily contaminated by moisture, oxygen, carbon dioxide, and other gaseous substances when exposed to air. As a result, the exposed surface does not exhibit their intrinsic catalytic activities. Up to now, conversions of most heterogeneous catalysts are not high enough to be used for the industrial scale biodiesel production. In comparison with homogeneous catalysts, relatively prolonged reaction periods are required in heterogeneous catalytic process. The Esterfif-H process is one of the few known processes which claims to have comparable performance as the homogeneous catalytic process.